This invention relates to a method and apparatus for cutting metal pieces, and more particularly to a method and apparatus for longitudinally cutting relatively long, hot metal pieces having a rectangular cross-section, such as continuous-cast steel slabs, into narrower widths.
Conventionally, steel products have been manufactured by following a sequence of operations as follows: first, molten steel refined in a steel-making furnace is poured into molds to solidify into ingots. The thus molded ingots are uniformly heated in a soaking pit, and then rolled into semi-finished steel. After cooling and surface conditioning, the semi-finished steel is reheated and rolled into the desired finished products.
In another process which has recently come into wider use, molten steel is continuously cast from the steel-making furnace directly into semi-finished shapes (or cast sections).
To increase the equipment utilization rate and productivity of the continuous casting process, it is necessary to decrease the variety of cast section sizes and increase the casting tonnage of each size to the greatest possible extent. Actually, however, rolled products, such as cold-rolled sheets, ordered by customers vary widely in thickness, width and so on.
To meet this demand by continuous casting, as many molds as are equivalent to the number of sizes, especially widths, ordered must be prepared, and such molds must be changed frequently. The frequent changing of the continuous casting molds, by the nature of things, takes up considerable time, thus reducing the operating time of the continuous caster. Then, a decrease in equipment utilization rate, equipment and labor productivity occurs.
There has been proposed a technique to overcome this problem by varying the width of the mold during the continuous casting operation. But to continuous-cast a slab with a width one-half the maximum possible width, for example, cuts the productivity in half. So this proposal is far from being a good solution.
To solve the problem more completely, as many different widths as possible should be grouped and rolled in a single piece, the width of which is that of the widest in the group, and the continuous cast piece then cut into the desired widths by some appropriate means.
In the conventional ingot producing process, various sizes of semi-finished steel products have been produced in the primary-mill stage. The continuous casting process, however, lacks this capacity to produce assorted sizes, except by changing the mold used therein. Thus, the practice has been to continuously cast a wide piece first, and then cut it into narrower widths later.
An example of a specific technique for carrying out this width-dividing operation is gas cutting in the longitudinal direction of the cast piece. Another example is disclosed in Japanese Patent Publication No. 83023 of 1976, in which a cast piece is passed between a pair of horizontal rolls each having an annular projection or projections therearound and disposed one above the other. As a consequence, a groove or grooves, having a V-shaped (or wedge-shaped) cross-section, are formed in the longitudinal direction of the piece. Then, the small thickness of metal left between the innermost points of the grooves in the top and bottom surfaces of the piece is gas-cut.
These methods make it possible to obtain semi-finished steel in various widths by longitudinally cutting a single width material. But gas cutting melts away a considerable amount of metal as slag, thereby decreasing the yield of the process. The low cutting speed results in the heat dissipating from the piece during cutting. This impedes the effective utilization of the sensible heat which the piece contains after casting and, therefore, reduces any saving of reheating energy for the subsequent rolling process. Gas cutting also exposes the coarse-grained structure within the cast section, which, being subsequently oxidized, remains as defects in the finished products.
Accordingly, still another method has been developed to cut a single width of material into two or more narrower widths without employing gas cutting. This method comprises cutting a groove or grooves having a V-shaped (or wedge-shaped) cross-section in each surface of the piece by passing the piece between a pair of horizontal rolls each having an annular projection or projections therearound and placed one above the other. By repeating this process, the top and bottom grooves are deepened gradually until the innermost points thereof meet to accomplish the cutting.
In longitudinally cutting a piece into three or more widths according to the last-described method, two variations are conceivable. A first method is to continue bisecting the widths until the desired number of sections are obtained. A second method is to cut the piece into the desired number of sections all at the same time. The first method has some shortcomings. When a piece is bisected, the halved sections each become cambered. It is very difficult, from a practical standpoint, to make longitudinal cut along such a cambered surface. Besides, the frequent passage through the cutting equipment lowers production efficiency. Even so, the second method surpasses the first in efficiency.
However, cutting a long piece of metal, such as a steel slab, longitudinally, into three or more narrower widths at one time, by using a pair of horizontal rolls having annular projections therearound, is also not without problems. A section through a piece having both sides of the original piece, i.e. a section to which the cutting force of the annular projection has not been applied, looks somewhat like an asymmetrical dogbone in cross-section, varying in thickness across the width. If such a piece is rolled through paired horizontal rolls for thickness reduction, the resulting product will have a large camber since the metal is elongated in varying amounts at different parts of the width. This camber not only makes the subsequent rolling difficult, but also lowers the quality of the finished product.
In cutting a flat piece of metal, such as a steel slab, with the paired rolls having annular projections therearound, care should be taken so that all cut sections have a symmetrical cross-section or their thickness irregularities across the width thereof should be minimized.